1. Field of the Invention
The present invention relates to a superconducting coil and, more particularly, to a superconducting coil capable of reducing a perpendicular magnetic field to the wide face of a superconducting wire by introducing a low current density coil into upper and lower ends of a main coil, in which the perpendicular magnetic field is most intense.
2. Description of the Related Art
As is well known, a superconductor industry, which is stood in the spotlight as a future technology, has been widely developed from superconducting materials to appliances, and therefore, advanced companies and countries have progressed in research and investment.
In particular, after a high temperature superconductor has been developed, its cooling method has been changed from a liquid helium cooling method to a liquid nitrogen cooling method. Therefore, it is anticipated that the high temperature superconductor will be very advantageous in economic and industrial viewpoint.
In the case of the high temperature superconducting electric devices using the high temperature superconductor, it is possible to make them have small size and weight and to optimize its efficiency in comparison with conventional electric devices.
In general, superconducting electric devices mostly have a main component having a coil shape, for example, a winding part of a transformer, a field coil of a rotator, an armature coil, and so on.
At this time, a superconducting coil used therein can conduct a high current in comparison with a copper coil, since there is no loss due to the zero resistance property of the superconducting coil and therefore it is possible to substantially increase current density.
As a result, the superconducting devices have weight and volume remarkably smaller than that of general electric devices.
However, as shown in a graph of FIG. 1 illustrating a correlation between a critical current density and a perpendicular magnetic field, a high temperature superconducting wire widely used nowadays is sensitive to a magnetic field essentially concomitant with a coil.
In particular, as shown in FIG. 1, the magnetic field perpendicular to the wire substantially decreases the critical current density, which is one of the characteristics of the wire, and increases alternating current loss.
Therefore, it was difficult to manufacture a high temperature superconducting coil capable of forming a high magnetic field. In order to solve the problem, a method of decreasing a cooling temperature of the superconductor is currently used.
That is, the critical current density is increased when the temperature of the superconductor is deceased as described above, and therefore, the critical current is also increased even when the same perpendicular magnetic field is applied.
Currently, a method of decreasing a cooling temperature of a high temperature superconductor to 25˜30 K (Kelvin temperature) to increase a critical current is used in several companies manufacturing superconducting electric devices. However, the method has problems of decreasing cooling efficiency, increasing cooling cost and therefore increasing overall price of the device, together with decreasing reliability of the device.